In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The ink-abhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
If a press is to print in more than one color, a separate printing member corresponding to each color is required. The original image is decomposed into a series of imagewise patterns, or "separations," that each reflect the contribution of the corresponding printable color. The positions of the printing members are coordinated so that the color components printed by the different members will be in register on the printed copies. Each printing member ordinarily is mounted on (or integral with) a "plate" cylinder, and the set of cylinders associated with a particular color on a press is usually referred to as a printing station.
To circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 disclose ablative recording systems that use low-power laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from the diode to the printing surface and focused onto that surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the surface layer). Other systems use laser energy to cause transfer of material from a donor to an acceptor sheet, to record non-ablatively, or as a pointwise alternative to overall exposure through a photomask or negative.
As discussed in the '617 and '092 patents, laser output can be generated remotely and brought to the recording blank by means of optical fibers and focusing lens assemblies. It is important, when focusing radiation onto the recording blank, to maintain satisfactory depth-of-focus--that is, the tolerable deviation from perfect focus on the recording surface. Adequate depth-of-focus is important to construction and use of the imaging apparatus; the smaller the working depth-of-focus, the greater will be the need for fine mechanical adjustments and vulnerability to performance degradation due to the alignment shifts that can accompany normal use. Depth-of-focus is maximized by keeping output beam divergence to a minimum.
Unfortunately, optical efforts to reduce beam divergence also diminish power density, since a lens cannot alter the brightness of the radiation it corrects; a lens can only change the optical path. Thus, optical correction presents an inherent tradeoff between depth-of-focus and power loss. U.S. Pat. No. 5,822,345 discloses an approach that utilizes the divergent output of a semiconductor or diode laser to optically pump a laser crystal, which itself emits laser radiation with substantially less beam divergence but comparable power density; the laser crystal converts divergent incoming radiation into a single-mode output with higher brightness.
The output of the laser crystal is focused onto the surface of a recording medium to perform the imaging function. In ablation-type systems, the beam is focused on the "ablation layer" of the recording material, which is designed to volatilize in response to laser radiation; again, the depth-of-focus of the laser beam provides a degree of tolerable deviation. In transfer-type systems, the beam is focused on the transfer layer. As used herein, the term "plate" or "member" refers to any type of printing member or surface capable of recording an image defined by regions exhibiting differential affinities for ink and/or fountain solution; suitable configurations include the traditional planar or curved lithographic plates that are mounted on the plate cylinder of a printing press, but can also include seamless cylinders (e.g., the roll surface of a plate cylinder), an endless belt, or other arrangement. Laser imaging is also widely used outside the context of lithography to produce, for example, color proofs and other graphic-arts products.
Practical imaging equipment requires lasers that respond nearly instantaneously to high-frequency square-wave power pulses so that imaging dots--that is, the spots produced by the laser beam on the recording material--appear as sharp, discrete, and ordinarily round shapes of consistent size. Dots must also be printed, or recording space left blank, at very closely spaced intervals to achieve typical print resolutions. Although the '470 application discloses the ability to control image-dot size by varying the pulse width within certain limits, it has been found that dot size can also change with the density at which dots are printed. The term "duty cycle" refers to the percentage of pixel locations in an imaged field that actually receive laser radiation (that is, the ratio of time during which the laser crystal is activated to the time it is inactive). The larger the duty cycle, the darker will be the resulting color, since in digital printing systems gray-scale densities or tints are achieved by varying pixel densities.
If the sizes of individual dots vary with the duty cycle, it will be impossible to establish consistent calibrations for color densities, since dot size also affects density. For example, if dots are smaller at low duty cycles, areas imaged at low pixel densities will print lighter than would be expected. And since documents typically contain regions of varying densities that may be interwoven in complex patterns, the problem cannot be corrected simply by altering the pixel density to correct for varying dot sizes.
A related imaging problem involves variation in distance between the laser output and the recording medium during the course of a scan. This is generally due to some mechanical misalignment in the imaging system, and tends to appear as a periodic condition (occurring, for example, due to eccentric rotation of the cylinder on which the recording medium is affixed during imaging). Although the underlying source of this problem lies in the mechanics of the imaging system rather than the response of the laser, variations in imaging distance and in laser power as a function of duty cycle tend to produce visually similar manifestations (in the form of varying spot sizes), and therefore create mutually reinforcing errors.